This invention relates to character overprinting in non-impact laser printers. Character overprinting includes but is not limited to character underlining.
Impact printers are often used as output devices for host computer systems. The host system transmits lines of data to the printer along with commands for each line to operate the printer. For each line of print, separate commands are available to print the line and advance the paper or to print the line without advancing the paper. If it is desired to underline selected characters in a line of character shapes, the line of character shapes is first transmitted along with a command to print without advancing the paper. The next line of data identifies which characters are to be overprinted with underlining, and it is accompanied by a command to print the underlining and then advance the paper. In impact printers, the ability to separately control the advance of paper and the printing of data for each line of data provides a capability for character overprinting.
Character overprinting as described above is impossible in high speed laser non-impact printers since the operations of printing and advance of the paper are not independent. In a typical laser non-impact printer, a periodically sweeping laser beam, in response to modulation signals from a character generator, images many parallel lines of data onto a continuously rotating cylindrical photoconducting drum. The images are developed on the drum as the drum rotates. When the drum rotates past a transfer station with the developed images thereon, the images are transferred onto a continuously moving sheet of paper which passes in close proximity to the surface of the drum. The paper is then discharged.
The host system transmits data with commands to a non-impact printer in much the same way as with an impact printer except that character overprinting such as underlining is difficult to attain for the reason mentioned above. One manner of accomplishing underlining would be to provide two sets of identical characters stored in a character generator memory, one with underlining. However, such an approach would require twice as much memory space in the character generator memory just to attain character underlining.